Skipping a complete
history of U.S. intelligence efforts, it is still helpful to get a feeling
for where the U.S. Navy stood as to intelligence on 7 December 1941.
At that time, the open intelligence agency responsible for providing intelligence
to the U.S. fleet was the Office of Naval Intelligence (ONI). ONI
basically consisted of a small administrative, analysis and reporting office
in Washington D.C., Naval Attaché’s at many embassies and consulates,
and District Intelligence Officers with very small staffs at all Naval
Districts and some commands. Despite these efforts, the strict secrecy
programs of the Japanese Navy contributed to the failure of ONI to fully
appreciate the enormous war making capabilities of the Imperial Japanese
Navy (IJN). Very little intelligence from prior decrypted IJN communications
was distributed beyond the top ONI officers or was permitted in ONI reports

Beginning about
January 1924 with limited funding, a naval capability to intercept, locate,
analyze and decrypt communications of potential enemies was started in
the offices of the Director of Naval Communications (DNC). This fledgling
organization was eventually organized as OP-20-G and was located in the
Navy Department offices in Washington D.C. It established intercept
and High Frequency Direction Finder (HFDF) sites in the Pacific, Atlantic
and continental U.S. as well as a Japanese telegraphic code school for
radio operators in Washington D.C. Decryption and analysis units
were established in the Pacific. In addition to intercepting naval
communications from Japanese, German and Italian navies, the Navy also
copied diplomatic messages of many foreign governments. The majority
of effort was directed towards Japan and included breaking the early Japanese
“Blue” book fleet code. Liaison was established with the Army’s counterpart,
Signals Intelligence Service (SIS) through its Washington headquarters
in the nearby Munitions Bldg. With the assistance provided by the
Navy’s break into both the prior M-1 machine cipher used by Japanese naval
attaches and other assistance, the Army was able to reconstruct a working
cipher machine to produce immediate plain language texts of the new Purple
diplomatic cipher used by the Japanese. Navy Lieutenant (j.g.) Francis
A. Raven solved the Purple key system so that a key only had to be recovered
every tenth day. The Navy constructed at least five of these machines.
An interesting development was that the fifth Purple machine that was earmarked
for the cryptanalysts at Station Hypo at Pearl Harbor was given to the
British in addition to a machine previously provided to London. One
wonders what the outcome of the Pearl Harbor attack would have been if
this machine had not been diverted from Pearl Harbor and that unit was
permitted to also decrypt the diplomatic messages leading up to the attack
on Pearl Harbor.

One ONI program
that greatly assisted OP-20-G’s Japanese decrypting and translation efforts
was the establishment of a resident language program in Japan for naval
officers. Many of these language officers eventually found there
way into OP-20-G billets in Washington, Pearl Harbor or the Far East.
One drawback of this program was that since most of these language officers
were naval academy graduates, they had to alternate their radio intelligence
work with tours of duty at sea in order to be promoted within the Navy’s
current policies. In fact, some of these officers were passed over
for promotion for lack of suitable sea experience despite the incomparable
value their work proved to be up to and throughout WWII.

In 1941, SIS
was primarily involved in intercepting and decrypting diplomatic traffic
with an emphasis on Japan and Germany. When the volume of Tokyo-Washington
diplomatic traffic increased with the Nomura talks beginning in mid-1941,
the decrypting of high level Japanese messages with the Purple cipher machines
was shared on alternating days between the Army and Navy. While the output
of all readable Japanese diplomatic decrypts was considerable, there were
not enough cryptographic personnel available in either service to timely
decrypt all Japanese diplomatic traffic in readable systems, even for sensitive
locations like Honolulu.

At this time,
SIS had a total of about 331 officers, men and civilians but not all were
engaged in cryptanalysis. As opposed to Navy practice, relatively
little effort was expended on the interception and decryption of Japanese
Army messages. OP-20-G had about 730 personnel throughout the world
for all purposes. Additional funds and the general military increases
had recently achieved even these small figures since the draft was instituted
in 1940. To get a perspective on the numbers required by war’s end to adequately
exploit Japanese and German messages over a fraction of their former geographical
areas, SIS had 7,000 personnel plus those in Australia and OP-20-G exceeded
8,000. In addition to its contribution of decrypting Japanese diplomatic
traffic, OP-20-G and Station Cast at Cavite/Corregidor had succeeded in
establishing the basic encryption method of JN-25, the code used by major
IJN fleet units and commands. Washington had recovered a number of stereotyped
messages in the second version, JN-25B but not on a timely basis.
A JN-25B key change on 1 December was a setback to this cryptanalytic effort.
The exclusive assignment of a little used IJN Admiral’s code to the Pearl
Harbor unit proved to be a terrible waste of some of the best cryptanalysts
the Navy had.

It is important
to emphasize the lack of any formal distribution procedures to inform responsible
fleet commanders of the intelligence information being gleaned from decrypts
of Japanese communications. In the Navy, this was complicated by
the self appointed intelligence expert of then Captain Richmond K. Turner
known as “Terrible Turner”, the new head of the Navy’s War Plans department
of CNO. The weakness of Admiral Stark as CNO let Turner completely
usurp the functions of ONI and DNC to fulfill their responsibilities to
properly warn fleet commanders of the impending Japanese actions based
on the Purple diplomatic decrypts and other indicators. More serious
war warning messages and a more accurate picture of the current situation
as indicated by Japanese decrypts that were advocated by Captain Laurence
Stafford as OP-20-G, Admiral Noyes DNC, and the acting Director of Intelligence
(DNI), Captain Kirk, were forestalled or greatly watered down by Turner.
One excuse Turner tried to give for such perfunctory warnings was that
Pearl Harbor had all the Japanese diplomatic decrypts, which was false.
Earlier, Captain Turner was convinced Japan would only attack Russia and
just before Pearl Harbor he convinced Stark that Japan was not ready to
attack the U.S. only the British. The new DNI Theodore S. Wilkinson
refused to challenge Turner’s rebuff of a further specific war warning
drafted by Captain Arthur H. McCollum on 5 December. Again on 6 December,
Stafford tried again but was dismissed by Noyes so as not to antagonize
Turner. On the Army side, General George G. Marshall and intermediaries
vetoed similar requests made by Colonels Rufus S. Bratton and Otis K. Sadtler.
Later, Marshall denied receiving the related decrypts. As Washington
politics go, both Stafford, Bratton and Sadtler were relegated to rather
minor posts and discredited, while Noyes and Turner were given prime advancement
billets and promotions. Although General Marshall was held to have
been derelict in his duties by the first Army board of inquiry on the Pearl
Harbor attack, the subsequent congressional investigation only found Admiral
Kimmel and General Short at fault for the Pearl Harbor disaster.
Marshall had the backing of both Secretary of War Stimson and President
Roosevelt. Stimson instigated a fierce campaign to reverse Marshall’s
prior dereliction finding. During the latter hearings, none of Turner’s
subordinates would break ranks and reveal Turner’s derelictions due to
his great wartime achievements and rank as Vice Admiral. Only subsequent
revelations have verified Turner’s and Marshall’s responsibility for impeding
more appropriate and timely warnings urged by intelligence professionals
based on Purple decrypts.